Resonators are used in many devices to allow detection of electromagnetic radiation of specific wavelengths. One type of device for detecting electromagnetic radiation is a spectrometer. Spectrometers are used in many applications for measuring properties of light across a range of wavelengths. For example, a spectrometer can be used for compositional analysis, by obtaining absorption or emission spectra for an object of interest. The presence and location of peaks within the spectra can indicate the presence of particular elements or compounds. Spectrometers are commonly used for analysis at optical wavelengths, but can also be used at other wavelengths such as microwave and radio wavelengths.
Spectrometers are typically relatively complex and expensive devices that require the alignment of a number of moving parts to be controlled with high precision. For example, a typical spectrometer may focus light onto a diffraction grating to split an incident beam into separate wavelengths, and the diffraction grating may be rotated to a specific angle to direct light of a particular wavelength towards a detector.
In recent years chip-based spectrometers have been developed which can be highly miniaturised, have no moving parts, and can be manufactured using well-established lithography techniques. WO2010128325 discloses a chip spectrometer without any moving parts. The chip spectrometer, which may be referred to as a spectrometer-on-a-chip, comprises a substrate onto which are patterned a waveguide and a plurality of disk resonators coupled to the waveguide. Light enters the waveguide at a first end and the resonators are arranged such that portions of the light in the waveguide are coupled into the disk resonators. Each resonator is arranged to support a resonant mode at a particular wavelength such that only light of that wavelength is coupled into the resonator. On top of each disk resonator is an electrode for detecting current that is proportional to the amount of light present in that resonator. The current detected in each resonator therefore indicates the amount of light at that wavelength that was present in the input beam of light. Each electrode is further connected to a signal bond pad for connecting the spectrometer to an external device for measuring the current. In WO2010128325, all the disk resonators work at the same resonance order and the larger the resonant mode wavelength of a disk resonator, the larger the size of the disk resonator. Moreover, the disk resonators are arranged on the substrate such that the larger the size of a disk resonator the further away from entrance to the waveguide the disk resonator is located.
The invention is made in this context.